Microscope exposes atoms' inner mysteries

Portland, Ore. -- A spherical-aberration corrector has enabled the transmission electron microscope at IBM's T.J. Watson Research Center (Yorktown Heights, N.Y.) to make the highest-resolution images in the world. Instead of blurry pictures of individual atoms, the researchers have obtained clear images of the individual molecular bonds among the different types of atoms in the crystalline lattice of a semiconductor surface.

"For a long time electron microscopes have been limited by the spherical aberration of electron lenses--we've had a correction lens for optical wavelengths, but not for electron lenses," said Cornell University professor John Silcox. Silcox made the unprecedented images with Phil Batson, an IBM researcher who recently designed and installed a second-generation spherical-aberration correction system made by Nion Co. (Kirkland, Wash.). As a result, the world's highest-resolution images are now made on IBM's 120,000-electron-volt (eV) scanning-tunneling electron microscope (STEM).

Feasibility demo"What we have made is a feasibility demonstration. Now that people know it's possible, those who really need to see atomic structure will try our approach," said Silcox.

The researchers clearly imaged a crystalline aluminum nitride surface, showing the hexagonal "wurtzite" arrangement of atoms. The crystalline aluminum nitride layer was fabricated to experiment with storing charge in aluminum-nitride/gallium-nitride/aluminum-nitride quantum wells. The gallium nitride behaves as a semiconductor, storing as little as one charge carrier, while the aluminum-nitride sandwich insulates the quantum well from electrodes above and below it.

The images clearly revealed for the first time the location and orientation of both the aluminum and the much tinier nitrogen atoms in the hexagonal wurtzite crystalline lattice pattern.

The researchers' aim was to help designers craft and troubleshoot nanoscale structures in increasingly small and thin semiconductor chips. For instance, future chips based on quantum wells only a few nanometers in diameter will work advantageously with surfaces of known polarization. Unfortunately, determining the polarization of an atomic layer on a chip usually means destroying it, and is sometimes impossible if the film is too thin to test. Such is the case for single-atomic "monolayers."

"Prior to our proof-of-concept demonstration, you had to use indirect methods to determine the polarization of a monolayer," said Silcox. "But with our instrument, you can look directly at the arrangement of atoms and see their polarization."

Now the group wants to start diagnosing problem chips by peering at their nanoscale structure for clues. Their special interest lies in III-V nitrides (such as gallium and indium nitrides), which hold the promise of enabling incredibly dense arrays of quantum wells to be fabricated on atomically thin monolayers. Unfortunately, such small devices in such extremely thin films will be difficult to troubleshoot. Dozens of faults could occur, all notoriously difficult to pinpoint using indirect measurements. Now faults can be directly inspected visually, to more easily diagnose why some work well and others do not.

"We want to continue to look at III-V nitrides, but next we want to try diagnosing quantum wells that seem to be misbehaving in unbeknownst ways, to see whether it has a stacking fault or one of many other types of possible problems," said Silcox. "We also want to look for faults in monolayers--single atomic layers--and [use the STEM] to see the structure of other kinds of atomic lattice patterns proposed for quantum wells."

The aberration corrector allows the STEM to focus a 0.9-angstrom-wide electron beam on a semiconductor surface, then collects the scattered electrons on a ring-shaped detector. Larger atoms deflect electrons more than smaller atoms, enabling their size and orientation to be clearly imaged. Using an imaging technique called "annular dark imaging," colors represent atomic shapes against a black background. For aluminum nitride, the big difference in size between the atoms makes molecules of the hybrid compound appear to be pear-shaped, with the larger aluminum atom at the thick end and the smaller nitrogen atom at the narrow end of each molecule.

Nion is now upgrading the electron microscope at Oak Ridge National Laboratory with a spherical-aberration correction system. Since the Oak Ridge STEM is 300 keV, compared with IBM's 120 keV, Nion predicts that Oak Ridge will have the highest-resolution electron microscope in the world once the conversion is completed next year. The multielement lens at the Oak Ridge lab will correct for both spherical and chromatic aberration in its STEM, enabling it to operate with subangstrom accuracy.

Silcox's research team included his colleagues at Cornell and his postdoctoral associate, Andre Mkhoyan.